The present invention relates to novel photochromic articles demonstrating a more consistent photochromic response over a broad temperature range and methods for preparing such articles. More particularly, this invention relates to photochromic articles having reduced temperature dependency. Temperature dependency is a particular problem in photochromic articles, e.g., spectacle lenses. The coloration of the photochromic article is effected by the temperature at which it is used. Typically, a photochromic article demonstrating the temperature dependency effect will not get dark enough when it is hot and/or will get too dark when it is cold.
All photochromic materials exhibit a change in absorbance, very typically with an observable change in color when exposed to electromagnetic radiation containing ultraviolet rays, such as the ultraviolet radiation in sunlight or the light of a mercury lamp. When the ultraviolet radiation is discontinued, thermally fading photochromic materials will return to their original (typically colorless) state. In such photochromic materials represented by, but not limited to, oxazines and naphthopyrans, the coloration reaction is largely photochemical while the decoloration (or fading) reaction is primarily thermal in nature. Photochromic materials that are operative in such a manner, will display what is called the “temperature dependency effect” in articles using these compounds.
Temperature dependency is a result of a shift in the equilibrium concentrations of the ground state or closed form (typically colorless) and activated state or open form (typically colored). At higher temperatures, the equilibrium will be shifted towards the ground (typically colorless) state while at lower temperatures, the equilibrium will be shifted towards the activated (typically colored) state. Articles will therefore present a different response depending on the ambient temperature.
In one approach, the temperature dependency of photochromic compositions and articles was reduced by switching to all naphthopyran photochromic compounds from the earlier used oxazines or mixtures of oxazines and naphthopyrans. While this improvement was substantial and welcomed, there remains a need to improve these and other photochromic systems by further reducing the temperature dependency effect.